By 1981 I'd suspect your Raleigh would have standard English thread bottom bracket cups, but this particular cup style did also come in Raleigh's proprietary 1-3/8" x 26tpi thread as well:



Any chance you could try threading a known, English thread cup into the shell? If it works, great, you have an English thread bottom bracket, and all is well in the universe. If it doesn't, then you need to explore other options, i.e. sourcing 1-3/8" x 26tpi cups or using a threadless cartridge.